Various problems associated with the generation and discharge of electrostatic charge during the manufacture and use of photographic film and paper products have been recognized for many years by the photographic industry. The accumulation of static charge on film or paper surfaces can produce irregular fog patterns in the sensitized emulsion layer(s). The presence of accumulated charge also can lead to difficulties in support conveyance as well as dust attraction to the support, which can result in repellency spots during emulsion coating, fog, desensitization, and other physical defects. The discharge of accumulated static charge during or after the application of sensitized emulsion layer(s) can produce irregular fog patterns or "static marks". The severity of static-related problems has been exacerbated greatly by increases in sensitivity of new emulsions, coating machine speeds, and post-coating drying efficiency. The generation of electrostatic charge during the film coating process results primarily from a tendency of high dielectric constant polymeric film base webs to undergo triboelectric charging during winding and unwinding operations, during conveyance through coating machines, and during finishing operations such as slitting and spooling. Static charge can also be generated during the use of the final photographic film product. In an automatic camera, winding roll film out of and back into the film cassette, especially in a low relative humidity environment, can produce static charging and result in marking. Similarly, high-speed automated film processing equipment can produce static charging that results in marking. Also, sheet films used in automated high-speed film cassette loaders (e.g., x-ray films, graphic arts films) are subject to static charging and marking.
One or more electrically-conductive antistatic layers can be incorporated into an imaging element in various ways to dissipate accumulated electrostatic charge, for example, as a subbing layer, an intermediate layer, and especially as an outermost layer either overlying the imaging layer or as a backing layer on the opposite side of the support from the imaging layer(s). A wide variety of conductive antistatic agents can be used in antistatic layers to produce a broad range of surface electrical conductivity. Many of the traditional antistatic layers used for imaging applications employ electrically-conductive materials which exhibit predominantly ionic conductivity, for example simple inorganic salts, alkali metal salts of surfactants, alkali metal ion-stabilized colloidal metal oxide sots, ionic conductive polymers or polymeric electrolytes containing alkali metal salts and the like. The electrical conductivities of such ionic conductors are typically strongly dependent on the temperature and relative humidity of the surrounding environment. At low relative humidities and temperatures, the diffusional mobilities of the charge carrying ions are greatly reduced and the bulk conductivity is substantially decreased. At high relative humidities an unprotected antistatic backing layer containing such an ionic conducting material can absorb water, swell, and soften. Especially in the case of roll films, this can result in the adhesion (viz., ferrotyping) and even physical transfer of portions of a backing layer to a surface layer on the emulsion side of the film (viz., blocking).
Antistatic layers containing electronic conductors such as conjugated conductive polymers, conductive carbon particles, crystalline semiconductor particles, amorphous semiconductive fibrils, and continuous semiconductive thin films or networks can be used more effectively than ionic conductors to dissipate charge because their electrical conductivity is independent of relative humidity and only slightly influenced by ambient temperature. Of the various types of electronic conductors disclosed in prior art, electronically-conductive metal-containing particles, such as semiconductive metal oxides, are particularly effective when dispersed with suitable polymeric binders. Antistatic layers containing granular, nominally spherical, fine particles of crystalline semiconductive metal oxides are well known and have been described extensively. Binary metal oxides doped with appropriate donor heteroatoms or containing oxygen deficiencies have been disclosed in prior art to be useful in antistatic layers for photographic elements, for example: U.S. Pat. Nos. 4,275,103; 4,416,963; 4,495,276; 4,394,441; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276; 5,122,445; 5,294,525; 5,382,494; 5,459,021; and others. Suitable claimed conductive binary metal oxides include: zinc oxide, titania, tin oxide, alumina, indium oxide, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Preferred doped conductive metal oxide granular particles include Sb-doped tin oxide, Al-doped zinc oxide, and Nb-doped titania. Additional preferred conductive ternary metal oxides disclosed in U.S. Pat. No. 5,368,995 include zinc antimonate and indium antimonate. Other suitable electrically-conductive metal-containing granular particles including metal borides, carbides, nitrides, and suicides have been disclosed in Japanese Kokai No. 04-055,492.
Antistatic backing or subbing layers containing colloidal "amorphous" vanadium pentoxide, especially silver-doped vanadium pentoxide, are described in U.S. Pat. Nos. 4,203,769 and 5,439,785. Colloidal vanadium pentoxide is composed of highly entangled microscopic fibrils or ribbons 0.005-0.01 .mu.um wide, about 0.001 .mu.m thick, and 0.1-1 .mu.m in length. However, colloidal vanadium pentoxide is soluble at the high pH typical of developer solutions for wet photographic film processing and must be protected by a nonpermeable, overlying barrier layer as taught in U.S. Pat. Nos. 5,006,451; 5,221,598; 5,284,714; and 5,366,855, for example. Alternatively, a film-forming sulfopolyester latex or a polyesterionomer binder can be combined with the colloidal vanadium oxide in the conductive layer to minimize degradation during processing as taught in U.S. Pat. Nos. 5,380,584;5,427,835; 5,576,163; 5,360,706; and others.
When an electroconductive layer is the outermost layer on a support, it must be to protected against abrasion or scratching which may occur during handling of the photographic element in order to avoid degradation of its antistatic performance. Since the back side of an imaging element typically has more opportunity to come into direct contact with equipment surfaces and with mechanical parts during manufacture, winding and unwinding operations, use in a camera, processing, and printing or projecting the processed photographic element, it is particularly liable to abrasion damage or scratching. Scratches and abrasion marks not only degrade image quality during printing and projection processes but also permanently damage processed photographic film. Numerous approaches to improving the resistance of the surface or outermost layers of photographic film to scratching and abrasion damage have been described in the prior art. As one of the more effective approaches, it is well known to provide at least one protective topcoat layer overlying the antistatic layer having physical properties such as increased hardness and reduced contact friction in order to enhance resistance to scratching and abrasion.
A photographic element having a conductive layer containing semiconductive tin oxide or indium oxide particles on the opposite side of the support from the silver halide sensitized emulsion layers with a polymer-containing intermediate backing layer overlying the conductive layer and an additional protective layer overlying the backing layer is disclosed in U.S. Pat. No. 5,026,622. The outermost protective layer includes gelatin, a matting agent, a fluorine-containing anionic surfactant, and dioctyl sulfosuccinate. Another conductive three-layer backing having an antistatic layer containing granular semiconductive metal oxide particles; an intermediate backing layer containing a latex of a water-insoluble polymer, matting agent, polystyrenesulfonate sodium salt, and gelatin; and an outermost protective layer containing at least one hydrophobic polymer such as a polyester or polyurethane, fluorine-containing surfactant(s), matting agent(s), and an optional slipping aid is described in U.S. Pat. No. 5,219,718. Further, a three-layer backing having an antistatic layer including conductive metal oxide granular particles or a conductive polymer and a hydrophobic polymer latex, gelatin, and an optional hardener is overcoated with an intermediate backing layer containing gelatin, a hydrophobic polymer latex, a matting agent, and backing dyes that is simultaneously overcoated with a protective layer comprising a fluorine-containing surfactant, a matting agent, gelatin, and optionally, a polymer latex is taught in U.S. Pat. No. 5,254,448. Photographic elements including such multi-layer backings were disclosed to retain antistatic properties after processing, exhibit acceptable transport performance against Teflon coated surfaces, and have good "anti-flaw" properties.
The use of small (&lt;15 nm) antimony-doped tin oxide particles having a high (&gt;8 atom %) antimony dopant level and a small crystallite size (&lt;100 .ANG.) in abrasion resistant conductive backing layers is claimed in U.S. Pat. No. 5,484,694. A multi-element curl control layer on the backside of the support wherein the conductive layer typically is located closest to the support, with an overlying intermediate layer containing binder and antihalation dyes, and an outermost protective layer containing binder, matte, and surfactant is also claimed.
Simplified two-layer conductive backings are taught in U.S. Pat. Nos. 5,366,855; 5,382,494; 5,453,350; and 5,514,528. An antistatic layer containing colloidal silver-doped vanadium pentoxide and a vinylidene chloride-containing latex binder or a polyester ionomer dispersion coated on the opposite side of the support from the silver halide emulsion layer and subsequently overcoated with a protective layer including a coalesced layer containing both film-forming and non-film-forming colloidal polymeric particles, optional cross-linking agents, matting agents, and lubricating agents is disclosed in U.S. Pat. No. 5,366,855. Such a protective layer was also disclosed to function as an impermeable barrier to processing solutions, to resist blocking, to provide good scratch and abrasion resistance, and to exhibit excellent lubricity. However, the addition of hard polymeric particles, such as poly(methyl methacrylate), to a film-forming polymer can produce brittleness in a coated layer. A photographic element containing an aqueous-coated antistatic layer containing conductive fine particles such as metal oxide particles, a butyl acrylate-containing terpolymer latex, and optionally, a hardening agent and a surfactant that is overcoated with a solvent-coated, transparent magnetic recording layer containing preferably nitrocellulose or diacetyl cellulose as the binder and carnauba wax as a lubricant is taught in U.S. Pat. Nos. 5,382,494 and 5,453,350. Similarly, an antistatic layer containing conductive metal oxide granular particles in a hydrophilic binder applied as an aqueous or solvent dispersion and overcoated with a cellulose ester layer optionally containing ferromagnetic particles is described in U.S. Pat. No. 5,514,528. A separate lubricating overcoat layer can be optionally applied on top of the cellulose ester layer.
The inclusion of lubricant particles of a specified size, especially those having a fluorine-containing polymer, in a protective surface or backing layer containing a dispersing aid or stabilizer, a hydrophilic or resin-type binder and optionally, crosslinking agents, matting agents, antistatic agents, colloidal inorganic particles, and various other additives is described in U.S. Pat. No. 5,529,891. Photographic elements incorporating such protective layers were disclosed to exhibit improved surface scratch and abrasion resistance as evaluated on a Taber Abrader.
Another method to improve the slipperiness and scratch resistance of the back surface of a photographic element is described in U.S. Pat. No. 5,565,311. The incorporation of slipping agents containing compounds having both a long-chain aliphatic hydrocarbon moiety and a polyether moiety as a solution, emulsion or dispersion preferably in a backing protective layer containing a film-forming binder and an optional crosslinking agent overlying an antistatic layer is reported to provide improved slipperiness and scratch resistance and reduce the number of coated layers in the backing. The addition of a matting agent can improve scratch resistance as well as minimize blocking of the emulsion surface layer or emulsion-side primer layer by the backing layer. Further, the inclusion of an antistatic agent, such as conductive metal oxide particles, in a backing protective layer containing slipping and matting agents and optionally, nonionic, anionic, cationic, or betaine-type fluorine-containing surfactants is disclosed in U.S. Pat. No. 5,565,311.
An abrasion-resistant protective overcoat including a selected polyurethane binder, a lubricant, a matting agent, and a crosslinking agent overlying a conductive backing layer is described in U.S. Pat. No. 5,679,505 for motion picture print films; the abrasion-resistant protective overcoat contains a crosslinked polyurethane binder and, thus, provides a nonpermeable chemical barrier for antistatic layers containing antistatic agents that are degraded by photographic processing such as colloidal vanadium pentoxide, semiconductive metal salts (vide U.S. Pat. Nos. 3,245,833; 3,428,451 and 5,075,171), conducting polymers such as crosslinked vinylbenzyl quaternary ammonium polymers (vide U.S. Pat. No. 4,070,189) or polyanilines (vide U.S. Pat. No. 4,237,194), as suggested in U.S. Pat. No. 5,679,505. Although U.S. Pat. No. 5,679,505 can provide certain advantages over conventional carbon black containing backing layers (described in U. S. Pat. Nos. 2,271,134 and 2,327,828), the use of a crosslinking agent in the topcoat (without which the conductivity of the preferred antistatic layer of colloidal vanadium pentoxide will be jeopardized) poses some manufacturing concerns for its practice: crosslinked polyurethanes of U.S. Pat. No. 5,679,505 may impose additional constraints on the composition and pot-life of the coating solutions as well as other manufacturing parameters; from a health and safety standpoint, some crosslinking agents may require special handling and disposal procedures; removal of a crosslinked polyurethane layer can hinder recycling of the support. Although the polyurethane topcoat disclosed in U.S. Pat. No. 5,679,505 can be useful for overcoating antistatic layers containing electroconductive metal oxide granular particles which do not require protection from photographic processing solutions, the high volume loading of metal oxide particles required to obtain adequate antistatic properties can degrade the physical properties of the backing. Also, metal containing semiconductive particles, in general are quite abrasive and can cause premature damage to finishing tools, such as, knives, slitters, perforators, etc. and create undesirable dirt and debris which can adhere to the imaging element causing defects.
An electrically-conductive single layer backing having a combination of electrically-conductive fine particles, such as conductive metal oxide granular particles, and particular gelatin-coated water-insoluble polymer particles is disclosed in European Patent Application No. 749,040 to provide both a high degree of conductivity at low volumetric concentrations of conductive particles and a high degree of abrasion resistance. The use of a combination of insoluble polymer particles and a hydrophilic colloid with conductive metal oxide fine particles to prepare electrically-conductive layers that require lower volume fractions of conductive particles than conductive layers prepared using only a hydrophilic colloid as binder is disclosed in U.S. Pat. No. 5,340,676. A similar beneficial result is disclosed in U.S. Pat. No. 5,466,567 for electrically-conductive layers in which a combination of a hydrophilic colloid and pre-crosslinked gelatin particles is used as the binder for the electroconductive fine granular particles. However, the abrasion resistance of such gelatin-containing layers is unsuitable, particularly for motion picture applications.
Electrically-conductive backing layers for use in thermally processable imaging elements are described in U.S. Pat. Nos. 5,310,640 and 5,547,821. As described in U.S. Pat. No. 4,828,971, backing layers useful for thermally processable imaging elements must provide adequate conveyance properties, resistance to deformation during thermal processing, satisfactory adhesion to the support, freedom from cracking and marking, reduced electrostatic charging effects, and exhibit no sensitometric effects. The use of electrically-conductive backings and protective overcoat layers for thermally processable imaging elements is described in U.S. Pat. No. 5,310,640. In one preferred embodiment, a protective layer containing polymethylmethacrylate as binder and a polymeric matting agent is positioned overlying a conductive layer containing silver-doped vanadium pentoxide dispersed in a polymeric binder. The use of a single-layer conductive backing having antimony-doped tin oxide granular particles, a matting agent, and a polymeric film-forming binder is taught in U.S. Pat. No. 5,547,821. Another preferred embodiment teaches the use of antimony-doped tin oxide granular particles in a conductive overcoat layer overlying the imaging layer. The reported Taber abrasion test results suggest that the relative level of abrasion resistance for the single-layer backings is inferior to that for the overcoated conductive backing layer described in U.S. Pat. No. 5,310,640. Also, surface scattering and haze is higher for single-layer conductive backings than for overcoated conductive backings. Further, from the surface resistivity and dusting data reported in U.S. Pat. No. 5,547,821, it can be concluded that it is particularly difficult to simultaneously obtain low dusting and high conductivity with single-layer conductive backings containing a polyurethane binder and granular electroconductive particles.
An electrically-conductive single-layer backing for the reverse side of a laser dye-ablative imaging element comprising electrically-conductive metal-containing particles, such as antimony-doped tin oxide particles, a polymeric binder, such as gelatin or a vinylidene chloride-based terpolymer latex, a matting agent, a coating aid, and an optional hardener is described in U.S. Pat. No. 5,529,884. Surface resistivity values (.apprxeq.9 log ohms/square) for the conductive backings were measured before and after the ablation process and exhibited virtually no change. No test data for abrasion or scratch resistance of the backing layers was reported.
As indicated hereinabove, the prior art for electrically-conductive backing layers and for abrasion and scratch resistant backing layers useful for imaging elements is extensive and a wide variety of multilayered backings have been disclosed. However, there is still a critical need in the art for protective backings which provide multiple functions such as electrical conductivity combined with abrasion and scratch resistance. In addition to providing electrical conductivity and abrasion and scratch resistance, backings should resist the effects of humidity change, not exhibit adverse sensitometric or photographic effects, strongly adhere to the support, exhibit low dusting, exhibit no ferrotyping or blocking behavior, provide adequate support conveyance characteristics during manufacture and use, be unaffected by photographic processing solutions, and still be manufacturable in an environmentally benign way at a reasonable cost. It is toward the objective of providing such improved electrically-conductive, abrasion and scratch resistant, backings that more effectively meet the diverse needs of imaging elements,--especially of silver halide photographic films but also of a wide range of other imaging elements--, than those of the prior art that the present invention is directed.
Electrically conducting polymers have recently received attention from various industries because of their electronic conductivity. Although many of these polymers are highly colored and are less suited for photographic applications, some of these electrically conducting polymers, such as substituted or unsubstituted pyrrole-containing polymers (as mentioned in U.S. Pat. Nos. 5,665,498 and 5,674,654), substituted or unsubstituted thiophene-containing polymers (as mentioned in U.S. Pat. Nos. 5,300,575; 5,312,681; 5,354,613; 5,370,981; 5,372,924; 5,391,472; 5,403,467; 5,443,944; 5,575,898; 4,987,042 and 4,731,408) and substituted or unsubstituted aniline-containing polymers (as mentioned in U.S. Pat. Nos. 5,716,550 and 5,093,439) are transparent and not prohibitively colored, at least when coated in thin layers at moderate coverage. Because of their electronic conductivity instead of ionic conductivity, these polymers are conducting even at low humidity. Moreover, these polymers can retain sufficient conductivity even after wet chemical processing to provide what is known in the art as "process-surviving" antistatic characteristics to the photographic support they are applied. Unlike metal-containing semiconducting particulate antistatic materials (e.g., antimony-doped tin oxide), the aforementioned electrically conducting polymers are less abrasive, environmentally more acceptable (due to absence of heavy metals), and, in general, less expensive.
However, it has been reported (U.S. Pat. No. 5,354,613) that the mechanical strength of a thiophene-containing polymer layer is not sufficient and can be easily damaged without an overcoat. Protective layers such as poly(methyl methacrylate) can be applied on such thiophene-containing antistat layers but these protective layers typically are coated out of organic solvents and therefore not highly desired. More over, these protective layers may be too brittle to be an external layer for certain applications, such as motion picture print films (as illustrated in U.S. Pat. No. 5,679,505). Use of aqueous polymer dispersions (such as vinylidene chloride, styrene, acrylonitrile, alkyl acrylates and alkyl methacrylates) has been taught in U.S. Pat. No. 5,312,681 as an overlying barrier layer for thiophene-containing antistat layers, and onto the said overlying barrier layer is adhered a hydrophilic colloid-containing layer. But, again, the physical properties of these barrier layers may preclude their use as an outermost layer in certain applications. The use of a thiophene-containing outermost antistatic layer has been taught in U.S. Pat. No. 5,354,613 wherein a hydrophobic polymer with high glass transition temperature is incorporated in the antistat layer. But these hydrophobic polymers reportedly may require organic solvent(s) and/or swelling agent(s) "in an amount of at least 50% by weight" of the polythiophene, for coherence and film forming capability.
As will be demonstrated hereinbelow, the present invention can provide an antistatic layer with a protective topcoat without the use of any crosslinking agent, to an imaging element, incorporating humidity independent, process-surviving antistatic characteristics as well as resistance to abrasion and scratching. Specifically, the present invention provides an antistatic layer comprising an electrically conducting polymer with or without a suitable film forming binder, and a polyurethane topcoat wherein the polyurethane has a tensile elongation to break of at least 50% and a Young's modulus measured at 2% elongation of at least 50000 psi, with certain advantages over the teachings of the prior art.